This invention relates to stereoscopy and more particularly to a method and apparatus for creating shadows which are perceived as having a three-dimensional or seemingly solid configuration. In one form of the invention, three-dimensional shadow images are utilized to facilitate the instrument navigation of aircraft or other vehicles where the visibility of landmarks is limited or absent.
The casting of a shadow by an object exposed to a source of light is a familiar phenomenon which has heretofore been used for practical purposes only to a very limited extent. The art of portraiture by silhouette is one example and the sundial is another. Shadows, sometimes animated, have been cast upon walls or screens for amusement and entertainment purposes.
Considered as an image, conventional shadows not only exhibit a total lack of detail, aside from a flattened outline of the subject, but are also essentially two-dimensional. Technically, a conventional shadow may have three dimensions if cast upon a curved or angled surface but this simply results in distortion of the image rather than adding any true depth perception as far as a viewer's reactions are concerned. As an image, the conventional shadow is very low in information content and presents a very unrealistic representation of a solid object. The conventional shadow of an object having a frontal appearance which differs from the rear appearance conveys no information as to whether the shadow is taken from the front or the back.
Prior systems for causing observers to perceive a more realistic three-dimensional representation of an object or scene require complex processing of the image data and in most cases are not capable of providing a live or immediate representation. Stereoscopic still pictures or 3D motion pictures and the more recently developed holographic techniques involve the projection of light through a film on which the image data must be previously recorded and stored. In each of these techniques, the 3D image is not created directly from the depicted objects but is instead reconstructed from data stored on film and a sizable period of time and considerable processing effort is needed for this purpose. While a live or immediate 3D television image presentation may be arranged by using dual cameras and other accessory equipment, in this case the image data must first be reduced to electronic signals and is then reconstructed into an optical image.
Considering another aspect of the background of the invention, instrument flight and instrument landing systems for aircraft and the like use radio signals from ground stations and signals produced aboard the aircraft by gyroscopic devices and the like to determine the lateral and vertical location of an aircraft relative to an airport or other geographical site and to determine the orientation of the aircraft about roll, pitch and yaw axes. This data is conventionally presented to the pilot through a series of visible dials, pointers, indicators, meters, and the like which may also advise if the aircraft is deviating from a desired course. This does not provide the pilot with a realistic visual depiction of the position and orientation of the aircraft relative to an airport or relative to the earth. Instead, the pilot must interpret the readings of the several instruments and mentally convert the data into a visualization of the situation of the aircraft. The pilot must then manipulate the controls of the aircraft in response to his understanding of the meaning of the various dials and indicators instead of reacting directly to visual perception of the earth and the airport. Conventional instrument landings on this basis are taxing and subject to serious misinterpretations of data by the pilot. Landing is generally considered to be the most hazardous stage of aircraft operation and this is particularly true when the landing approach must be made blind on the basis of conventional instrument readings. This is a common situation where clouds or fog obscure the landing site.